Type inference by the compiler in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is type inference in Swift?

Type inference is when the Swift compiler automatically figures out the type of a variable or constant based on the value you assign to it, so you don't have to write the type explicitly.

beginner

How does Swift infer the type of <code>let number = 10</code>?

Swift sees the value 10 is an integer, so it infers the type of number as Int.

beginner

Can you give an example where type inference helps make code simpler?

Instead of writing <code>let name: String = "Alice"</code>, you can write <code>let name = "Alice"</code>. Swift infers <code>name</code> is a <code>String</code>.

intermediate

What happens if Swift cannot infer the type?

If Swift cannot figure out the type from the value, it will give an error and ask you to specify the type explicitly.

intermediate

Does type inference affect performance at runtime?

No, type inference happens only at compile time. The compiled program runs with explicit types, so there is no performance cost.

What type does Swift infer for let pi = 3.14?

A Double

B Int

C String

D Float

Swift infers decimal numbers like 3.14 as Double by default.

Which of these requires explicit type annotation in Swift?

A let count = 5

B let message = "Hello"

C let emptyArray = []

D let isActive = true

An empty array has no value to infer type from, so you must specify its type explicitly.

What is the benefit of type inference?

A Automatically fixes bugs

B Slows down the program

C Removes the need for variables

D Makes code shorter and easier to read

Type inference helps write less code and keeps it clear without losing type safety.

If you write let value = 42, what type does Swift assign to value?

A String

B Int

C Double

D Bool

42 is an integer, so Swift infers the type Int.

What happens if you try to assign a different type to a variable after type inference?

A Swift throws a compile-time error

B Swift allows it without error

C Swift changes the variable's type automatically

D Swift ignores the new value

Swift enforces type safety, so assigning a different type causes a compile-time error.

Explain what type inference is and why it is useful in Swift programming.

Think about how Swift guesses the type from the value you assign.

Describe a situation where you must specify the type explicitly even when using type inference.

When there is no value to guess the type from.

Practice

1. What does type inference mean in Swift?

easy

A. The program runs slower because types are guessed.

B. You must always write the type explicitly.

C. The compiler automatically figures out the variable's type.

D. You cannot use variables without specifying types.

Solution

1. Step 1: Understand type inference concept

   Type inference means the compiler guesses the type of a variable based on the value assigned.

2. Step 2: Compare options with definition

   Only The compiler automatically figures out the variable's type. correctly describes automatic type detection by the compiler.

3. Final Answer:

   The compiler automatically figures out the variable's type. -> Option C

4. Quick Check:

   Type inference = automatic type detection [OK]

Hint: Type inference means compiler guesses type automatically [OK]

Common Mistakes:

• Thinking you must always write types explicitly
• Believing type inference slows down the program
• Confusing type inference with dynamic typing

2. Which of the following Swift variable declarations uses type inference correctly?

easy

A. let number = 10

B. let number: = 10

C. let number Int = 10

D. let number : Int 10

Solution

1. Step 1: Check syntax for variable declaration with type inference

   Type inference allows omitting the type, so let number = 10 is correct.

2. Step 2: Identify syntax errors in other options

   Options B, C, and D have syntax errors: missing type name, wrong order, or missing assignment operator.

3. Final Answer:

   let number = 10 -> Option A

4. Quick Check:

   Correct syntax with inference = let number = 10 [OK]

Hint: Look for correct Swift syntax without explicit type [OK]

Common Mistakes:

• Adding colon without type name
• Writing type before variable name incorrectly
• Omitting assignment operator

3. What is the type of variable value in this Swift code?

let value = 3.14

medium

A. Double

B. Int

C. String

D. Float

Solution

1. Step 1: Identify the literal type assigned

   The number 3.14 is a decimal number, which Swift treats as Double by default.

2. Step 2: Confirm Swift default for floating-point literals

   Swift infers floating-point literals as Double unless specified otherwise.

3. Final Answer:

   Double -> Option A

4. Quick Check:

   3.14 literal = Double type [OK]

Hint: Decimal numbers default to Double in Swift [OK]

Common Mistakes:

• Assuming 3.14 is Float by default
• Confusing Int with floating-point types
• Thinking literals default to String

4. Identify the error in this Swift code snippet:

var name = "Alice"
name = 42

medium

A. No error, type inference allows changing type.

B. Error: Cannot assign Int to a variable inferred as String.

C. Error: Missing semicolon after first line.

D. Error: Variable must be declared with explicit type.

Solution

1. Step 1: Check initial type inferred for variable

   Variable name is assigned a String "Alice", so compiler infers type String.

2. Step 2: Analyze reassignment with different type

   Assigning an Int (42) to a String variable causes a type mismatch error.

3. Final Answer:

   Error: Cannot assign Int to a variable inferred as String. -> Option B

4. Quick Check:

   Type mismatch on reassignment = Error [OK]

Hint: Variable type fixed by first assignment, cannot change later [OK]

Common Mistakes:

• Thinking type inference allows changing variable type
• Assuming semicolons are required in Swift
• Believing explicit type declaration is always needed

5. Given this Swift code, what is the type of result?

let numbers = [1, 2, 3]
let result = numbers.map { $0 * 2 }

hard

A. Array<Any>

B. Int

C. [Double]

D. [Int]

Solution

1. Step 1: Understand the type of numbers

   Array numbers contains Int values, so its type is [Int].

2. Step 2: Analyze the map operation and type inference

   The map multiplies each Int by 2, producing another Int for each element, so the result is an array of Ints.

3. Step 3: Confirm inferred type of result

   Since map returns an array of transformed elements, result is inferred as [Int].

4. Final Answer:

   [Int] -> Option D

5. Quick Check:

   map on [Int] with Int output = [Int] [OK]

Hint: map returns array of same type as transformed elements [OK]

Common Mistakes:

• Thinking map returns a single Int
• Assuming map changes type to Double automatically
• Believing result is Array<Any> without type inference